1. Field of the Invention
The invention relates to a method of heteronuclear decoupling in magnetic resonance spectroscopy, where magnetic resonance signals originating from a first type of nucleus in an object are generated at least by irradiating the first type of nucleus with an RF electromagnetic excitation pulse, the first type of nucleus being spin-coupled to a second type of nucleus in the object, a pulse train of inversion pulses being applied to the second type of nucleus at least during signal acquisition of the magnetic resonance signals, in order to decouple the first type of nucleus from the second type of nucleus.
The invention also relates to a device for determining a spectrum from at least one magnetic resonance signal, which device comprises means for generating a steady, uniform magnetic field, first transmitter means for transmitting RF electromagnetic excitation pulses for exciting a first type of nucleus in an object, second transmitter means for transmitting inversion pulses to a second type of nucleus in the object, said second type of nucleus being spin-coupled to the first type of nucleus, in order to decouple the first type of nucleus from the second type of nucleus, receiver means for receiving resonance signals from the object, means for generating at least one magnetic field gradient, sampling means for sampling the magnetic resonance signal, and display means for displaying the spectrum, and also comprises processing means which include programmed arithmetic means for determining the spectrum from sampling values obtained by means of the sampling means, the programmed means also being suitable for activating, at least during signal acquisition of the resonance signals, the second transmitter means in order to transmit the inversion pulses.
2. Prior Art
A method of this kind is particularly suitable for spatially localized spectroscopy or spectroscopic imaging, and can be used inter alia for in vivo .sup.13 C and .sup.31 P spectroscopy.
Such a method is known from U.S. Pat. No. 4,470,014. According to such a method, an object is arranged in a steady, uniform magnetic field and, in order to obtain magnetic resonance signals from a first type of nucleus, for example a .sup.13 C isotope, RF electromagnetic excitation pulses are applied to the object via first transmitter means, which pulses have a frequency contents around the magnetic resonant frequency of the first type of nucleus. The first type of nucleus is molecularly coupled to a second type of nucleus, for example .sup.1 H hydrogen; depending on the kind of molecule, the first type of nucleus may be spin-coupled to one or to more than one nucleus of the second type. The way of coupling in a molecule has consequences as regards a spectrum to be formed from the resonance signals, for example by Fourier transformation. A single coupling will produce a doublet in the spectrum when the second type of nucleus .sup.1 H, which means that two resonance peaks will occur for the relevant molecule. This is due to a nuclear spin 1/2 of hydrogen (spin up or spin down), so that part of the nuclear spin population of the second type of nucleus in the relevant module will experience a chemical environment which is slightly different from that experienced by a complementary, substantially identical part. The magnetic resonant frequency of the parts, therefore, will differ slightly. Multiple coupling will give rise to multiplets in the spectrum. In order to obtain a higher resolution and a higher signal-to-noise ratio, during signal acquisition of the magnetic resonance signals, the nuclei of the second type of nucleus in the known method are irradiated, using second transmitter means, with a pulse train of inversion pulses having a frequency contents around the magnetic resonant frequency of the second type of nucleus. The inversion pulses are composite pulses having a varying phase pattern. For the conditions in which decoupling of the nuclei of the first and the second type is realized, reference is made to said United States Patent Specification. As a result of decoupling, a doublet or a multiplet in a non-decoupled spectrum will appear as a single resonance peak in the decoupled spectrum, the intensity thereof being the sum of intensities of individual peaks. The spectrum obtains a higher resolution, which means that various molecules having the first type of nucleus can be distinguished better and that the signal-to-noise ratio is higher. Inter alia in the case of spatially localized spectroscopy, where within the object a volume part is selected for which a spectrum is determined, in order to achieve suitable decoupling across the entire volume of importance it is necessary that the inversion pulses are homogeneous, which means that the strength of the RF electromagnetic field should be substantially the same throughout the transmitter coil. For example, when use is made of second transmitter means which include a transmitter coil around a head of the object or a transmitter coil around the entire object, said homogeneity requirement will usually be satisfied. When use is made of a surface coil, only locally delivering a field of sufficient strength, the required uniform sensitivity pattern will usually not be achieved. In that case the inversion pulses in accordance with the known method are not capable of achieving suitable decoupling across the entire sensitivity range of the coil. Even for nuclei such as phosphor where the very near .sup.31 P-.sup.1 H coupling (5-10 Hz) starts to play a dominant role as regards resolution of the spectrum at a comparatively low field strength (1.5 T), it is necessary to achieve population inversion over a non-uniform sensitivity pattern.